1. Field of the Invention
This invention relates to a highly crystalline polypropylene. More particularly it relates to a polypropylene having an extremely high crystallinity even when any additive for secondarily enhancing its crystallinity such as nucleating agent, etc. is not added.
2. Description of the Related Art
Crystalline polypropylene (hereinafter referred to as "PP") as a thermoplastic resin is provided with general-purpose properties due to which it is usable as parts of automobiles, domestic appliances, fibers, food-packaging materials, etc. and excellent physical properties (such as mechanical strength, stiffness, heat resistance, chemical resistance, electrical properties, etc.), and also high impact resistance can be imparted thereto in addition to the above physical properties by block-copolymerization; hence in recent years its demand has been remarkably increasing.
However, these properties have not yet been fully satisfactory depending on its applications and its use has been restricted. For example, the stiffness of injection-molded products obtained from PP, particularly the stiffness thereof in a high temperature region in the vicinity of 80.degree. C., is inferior to those of molded products obtained from polystyrene, ABS resin, etc. and also its heat deformation-resistant temperature is low. Further, in the fields of film and fibers, too, polyester resins, etc. are superior to PP in the aspect of nerve. Thus, if it is possible to impart to a very cheap PP, the same extent of properties in the aspect of stiffness as that of these resins or impart thereto stiffness filling in the gap between PP and these resins, then it can be expected that use of PP for industrial products or industrial parts around automobiles, domestic appliances, etc. will be more extended. By enhancing the stiffness, it is possible to make its molded products thinner while the same strength is retained, which serves resources-saving. In the field of packaging of foods, etc., since highly stiff materials have small shrinkage even at high temperatures, it is possible to shorten the drying time of printing ink on the surface of film, etc. by raising its secondary processing temperature to thereby improve the operation efficiency due to speed-up on the production line. In the field of fibers such as cotton for bed-clothes, carpet, etc., by increasing the bulkiness of fibers, such effects can be expected that the feeling of fibers is improved, the elastic recovery properties are enhanced and the decrease in resilience are reduced. Any of these matters can be achieved by enhancing the stiffness of PP.
As to the prior art for enhancing the stiffness of molded products of PP, for example, there is a process of adding an organic nucleating agent such as aluminum p-tert.-butylbenzoate, 1,3,2,4-dibenzylidenesorbitol, etc., to PP, followed by molding the mixture. Further, there is another process of adding various inorganic fillers such as talc, calcium carbonate, mica, barium sulfate, asbestos, calcium silicate, etc., followed by molding. However, the former process has drawbacks that the cost not only becomes high, but also luster, impact strength, tensile strength, elongation, etc. are reduced, while the latter process has drawbacks that the light-weight properties and transparency each specific of PP are not only damaged, but also the impact strength, luster, appearance, tensile elongation, processability, etc. are reduced. Further, processes of obtaining highly stiff PP without adding any nucleating agent, inorganic filler, etc. are disclosed in Japanese patent application laid-open Nos. Sho 57-47305/ 1982, Sho 59-22131/1984, etc., but any of PPs disclosed therein are not crystalline enough to be able to develop a high stiffness.
Thus the present inventors have made extensive research on production of a PP whose molded products have a high stiffness even when a high stiffness-effecting promoter such as fillers (e.g. talc) or additives (e.g. nucleating agent) is not added and even when conventional molding conditions, not particular molding conditions, are applied, namely a PP having a primary structure exhibiting a high crystallinity. As a result, we have found that a PP having specified properties exhibits a never observed high crystallinity and when molded into various molded products, developing a high stiffness, and have completed the present invention based on this finding.